Cherry-pick from stable/liberty: Fix for: OPENSTACK-1111
Cherry-pick from stable/liberty: Fix for: OPENSTACK-1111

refactored custom backhaulvnid code to use the common update_router code.

Change-Id: I7831db68982a651e1967e833ca19177ac255ce6e
Closes-Bug: OPENSTACK-1111

naveensan1/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron,naveensan1/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron